Medical Flight Tests were always conducted by CAA Staff or FIEs. There were no written instrucrtions or any guidance documents available, those who did it discussed the requirements with the CAA Doctor and got on with it. Invariably it required the examiner to devise a test to match the specific medical requirement. It has nothing to do with obesity information sheets. In the case of BMI its simply a test of the candidates functional reach, can they reach all controls and operate them correctly i.e. they are not impaired by their size and if they are to what extent?
I think the reason it was confined to a small group of examiners was to make it easier for the CAA doctors who knew who they were dealing with, whilst those doing it didn't have to keep asking questions. If you don't know what to do, then perhaps you should not be doing it.